1. Field of the Invention
The present invention relates to a hydraulic pressure control device for controlling the delivery pressure of a pump in accordance with maximum load pressure of actuators.
2. Description of the Related Art
FIGS. 5 and 6 show an example of a conventional hydraulic pressure control device.
As can be seen from FIG. 5, this hydraulic pressure control device has a dual valve construction incorporating a pair of directional control valves 2 of the same structure and circuit configuration. The following discussion depicts the structure of one of the directional control valves 2 and its associated circuit.
Referring to FIG. 5, a pump P is connected to a pump port 3 of the directional control valve 2 through a supply line 1. The supply line 1 remains always connected to conduct a fluid regardless of whether the directional control valve 2 is set to its neutral position or switched to a right or left position, and a far end of the supply line 1 is closed.
An unillustrated actuator is connected to a pair of actuator ports 4, 5 of the directional control valve 2. These actuator ports 4, 5 are closed when the directional control valve 2 is in its neutral position.
The directional control valve 2 also has a interconnecting port 7 which is connected to an inflow side of a pressure compensating valve 6. The interconnecting port 7 is closed when the directional control valve 2 is set to its neutral position, and is connected to the pump port 3 when the directional control valve 2 is switched to its right or left position. The opening of a variable throttle 8 formed between the interconnecting port 7 and the pump port 3 is determined such that the opening becomes proportional to the travel of switching of the directional control valve 2.
An outflow side of pressure compensating valve 6 is connected to a connecting port 9 of the directional control valve 2, with a check valve 10 provided between the pressure compensating valve 6 and the connecting port 9 to allow flow from the pressure compensating valve 6 to the connecting port 9 only.
The connecting port 9 is connected to a tank port 11 when the directional control valve 2 is set to its neutral position, to the actuator port 5 when the directional control valve 2 is switched to the left position, and to the actuator port 4 when the directional control valve 2 is switched to the right position of FIG. 5.
A pilot pressure is taken out from a line between the connecting port 9 and the tank port 11, the actuator port 4 or the actuator port 5.
One of pilot pressures thus obtained from the directional control valves 2 whichever higher is selected by a shuttle valve 12 and led to a pilot line 13.
The selected higher pilot pressure is led to a pilot chamber at one end of the pressure compensating valve 6. One the other hand, a pressure on the upstream side of the relevant pressure compensating valve 6 is led to a pilot chamber at the other end of the pressure compensating valve 6.
The pressure compensating valve 6 thus configured serves to keep the pressure on its upstream side higher than the pilot pressure introduced from the pilot line 13 by a pressure differential corresponding to an elastic force exerted by a spring 14.
The selected higher pilot pressure is also led to a pilot chamber at one end of a regulator valve 15 through the pilot line 13. One the other hand, pump delivery pressure is led to a pilot chamber at the other end of the regulator valve 15.
The regulator valve 15 thus configured serves to generate a control pressure from the pump delivery pressure in accordance with the pilot pressure of the pilot line 13 and the pump delivery pressure. When the control pressure is supplied to a regulator 16, the regulator 16 controls the angle of inclination of a rotary member of the pump P and keeps its delivery pressure higher than the pilot pressure by a specified amount.
Operation of this hydraulic pressure control device is now described.
When the directional control valve 2 is in its neutral position, the actuator ports 4, 5 are closed and, thus, a current load of the actuator is maintained as it is. Since the connecting port 9 is connected to the tank port 11 and the internal pressure of the pilot line 13 is equal to a tank pressure in this case, the pump delivery pressure is maintained at a relatively low standby pressure which is higher than the tank pressure by a specified amount.
If the directional control valve 2 is switched to the left position of FIG. 5, the pump port 3 is connected to the interconnecting port 7. In this case, the pump delivery pressure is controlled by the variable throttle 8 and led to the relevant actuator by way of the pressure compensating valve 6, the connecting port 9 and the actuator port 5 in this order.
One the other hand, return oil from the actuator is returned to the tank port 11 through the actuator port 4. Thus, the actuator is driven. At this point, the maximum load pressure among load pressures of individual actuators is selected by the shuttle valve 12 and led to the pilot line 13.
The pump delivery pressure or the pressure on the upstream side of the variable throttle 8 is maintained at a pressure which is higher than the maximum load pressure of the individual actuators by a specified amount by the regulator valve 15 and the regulator 16.
At the same time, the pressure on the upstream side of the pressure compensating valve 6 or the pressure on the downstream side of the variable throttle 8 is maintained at a pressure which is higher than the pilot pressure by the pressure corresponding to the elastic force exerted by the spring 14.
This means that the pressure differential between the upstream side and the downstream side of the variable throttle 8 is kept constant. It is therefore possible to maintain a constant actuator speed if the opening of the variable throttle 8 is determined in accordance with the travel of switching of the directional control valve 2 and the rate of flow through the variable throttle 8 is thereby determined.
Operation of the hydraulic pressure control device is simply reversed when the directional control valve 2 is switched in the right direction as illustrated. Thus, a detailed description of this case is omitted.
FIG. 6 shows a more specific example of the directional control valve 2, the pressure compensating valve 6 and the check valve 10 of the aforementioned conventional hydraulic pressure control device.
A spool 19 is slidably fitted in a spool hole 18 formed in a valve body 17.
A pair of tank ports 11a are formed at both sides of the valve body 17 and a pair of actuator ports 4, 5 are provided on the inside of the tank ports 11a. Further, a pair of connecting ports 9 are formed on the inside of the actuator ports 4, 5. Accordingly, when the spool 19 is switched to its right or left direction, through an annular groove 20, one of the actuator ports 4, 5 is connected to its corresponding connecting port 9 and the other is connected to its corresponding tank port 11a.
The connecting ports 9 are individually connected to a pilot line 13 which is not illustrated. Tank ports 11b are formed also on the inside of the connecting ports 9 so that the connecting ports 9 are connected to the respective tank ports 11b when the spool 19 is set to its neutral position as shown in FIG. 6.
A interconnecting port 7 is formed approximately in the middle of the valve body 17 and a pair of pump ports 3 are provided on both sides of the interconnecting port 7. Thus, the interconnecting port 7 is connected to the pump ports 3 no matter whether the spool 19 is switched in its right or left direction. As described earlier in connection with FIG. 5, a variable throttle 8 is formed between the interconnecting port 7 and each pump port 3 and the opening of the variable throttle 8 is determined such that the opening becomes proportional to the travel of switching of the spool 19.
The directional control valve 2 thus constructed integrally incorporates the pressure compensating valve 6 and the check valve 10.
An assembly hole 22 perpendicular to the spool hole 18 is formed approximately in the middle of the valve body 17, a lower end of the assembly hole 22 being connected to the interconnecting port 7. Right and left side portions of the assembly hole 22 are connected to the right and left connecting ports 9 through a pair of passages 23, respectively.
A movable sleeve 44 is slidably fitted in the assembly hole 22, an upper end of the movable sleeve 44 being closed by a closing member 24.
Further, a poppet 25 is fitted in the movable sleeve 44. The internal pressure of the passages 23 is led to a back pressure chamber of the poppet 25 through connecting holes 21 formed in right and left side portions of the movable sleeve 44 and the poppet 25.
The poppet 25 is brought into contact with a seating surface formed within the movable sleeve 44 by a spring 26 provided between the closing member 24 and the poppet 25. In this condition, control holes 27 formed on both the right and left side portions of the movable sleeve 44 are cut off from the interconnecting port 7 by the poppet 25.
A cover 28 is fixed onto the valve body 17 covering the movable sleeve 44 and the closing member 24.
The closing member 24 is disposed so that its upper end faces a pilot chamber 29 formed in the cover 28 and an elastic force of a spring 14 is exerted on the upper end of the closing member 24. As a result, the movable sleeve 44 is forced against the lower end of the assembly hole 22 by the elastic force of the spring 14. In this condition, the control holes 27 formed on the right and left side portions of the movable sleeve 44 are cut off from the passages 23.
A pilot pressure taken from the pilot line 13 is introduced into the pilot chamber 29.
It is now assumed that the spool 19 has just been switched from the neutral position shown in FIG. 6 in a direction shown by an arrow k.
As the pump ports 3 are connected to the interconnecting port 7 with their opening corresponding to the travel of switching of the spool 19 in this case, oil delivered from a pump is introduced into the assembly hole 22 and acts on a lower end of the movable sleeve 44 and the poppet 25. Consequently, the movable sleeve 44 moves upward overwhelming the pushing force of the spring 14 and the control holes 27 open to the passages 23. At this point, the pump delivery pressure causes the poppet 25 to come apart from the seating surface and the pump delivery pressure controlled by the control holes 27 is led to the connecting ports 9 through the passages 23.
In this case, both of the connecting ports 9 are cut off from the tank ports 11b while the connecting port 9 beside the actuator port 5 is connected to the actuator port 5 through an annular groove 20. Control pressure led to this connecting port 9 is thus supplied to an unillustrated actuator through the actuator port 5. Since the actuator port 4 is connected to the left-hand tank port 11a through another annular groove 20 at the same time, return oil from the actuator is discharged through the actuator port 4 and the left-hand tank port 11a.
As described earlier, one of load pressures of individual actuators led to the connecting ports 9 whichever higher is selected by the shuttle valve 12 and introduced into the unillustrated pilot line 13 at this point.
The maximum load pressure of one actuator is then led to the pilot chamber 29, whereby the position of the movable sleeve 44 is determined in accordance with the pressure at the interconnecting port 7 and the maximum load pressure of the actuator. Thus, the pressure at the interconnecting port 7 is controlled according to the current opening of the control holes 27 and kept higher than the pilot pressure by a pressure differential corresponding to the elastic force exerted by the spring 14.
The pump delivery pressure at the pump ports 3 is kept higher than the maximum load pressure of the actuators by a specified amount by a regulator valve 15 and a regulator 16 as described earlier.
This means that the pressure differential between the upstream side and the downstream side of the variable throttle 8 is kept constant. It is therefore possible to maintain a constant actuator speed if the opening of the variable throttle 8 is determined in accordance with the stroke of the spool 19 and the rate of flow through the variable throttle 8 is thereby determined.
Since the pilot line 13 is connected to the relevant tank port 11 when the directional control valve 2 is in its neutral position in the above-described conventional hydraulic pressure control device, the pump delivery pressure is kept at a relatively low standby pressure. When the directional control valve 2 is switched from the neutral position to another position, the pilot line 13 is cut off from the tank port 11 and connected to the actuator port 4 or 5 so that the maximum load pressure among load pressures of the individual actuators is selected and introduced.
It is to be noted, however, that if the hydraulic pressure control device is constructed such that the actuator ports 4, 5 are connected to the connecting ports 9 before the pump ports 3 are connected to the interconnecting port 7 when the directional control valve 2 has been switched, the actuator ports 4, 5 are connected to the pilot line 13 which has thus far been kept at the tank pressure during that time lag. A consequence of this is that an instantaneous flow occurs until the pilot line 13 is filled with pressurized oil, causing the load pressure of each actuator to be released into the pilot line 13. In a case where a cylinder is used as an actuator, for example, a load pressure applied to a bottom-side chamber of the cylinder will be introduced into the pilot line 13 when the directional control valve 2 is switched to increase the load pressure, and this causes a shock such as an instantaneous load reduction.
On the contrary, if the hydraulic pressure control device is constructed such that the actuator ports 4, 5 are connected to the connecting ports 9 after the pump ports 3 are connected to the interconnecting port 7 when the directional control valve 2 has been switched, the pressure in the connecting ports 9 will be brought to the pump pressure during the time lag and the pump pressure will be introduced to the pilot line 13. As a consequence, the pump pressure will be increased than the pump pressure in the pilot line 13 by a specified amount by the regulator valve 15 and the regulator 16, eventually causing a rapid increase in the pump delivery pressure. The occurrence of such a pressure boost phenomenon involving the rapid increase in the pump delivery pressure will cause irregularity in operation or energy losses. Also when a sudden increase in pump delivery pressure used to cause actuator startup shocks.